1. Field of the Invention
The present invention concerns data encryption or scrambling and relates in particular to the encryption of analogue data, such as speech, and the subsequent reconstitution of the scrambled data into its original form. One field where scrambling is of importance is radio as radio signals can be picked up by anyone with a correctly tuned receiver.
2. Description of Related Art
A well known encryption technique employed for speech signals involves sampling and digitising the signal to be scrambled with a delta modulator. The digitised signal is fed into a storage device at one rate and read out of the device at a second, different rate. In effect the original signal is alternately compressed or expanded in time. However this technique alone does not provide a very high level of security. Any receiving device which can reproduce the inverse of the two encryption clock rates will be able to reconstruct the data.
Previously protection from one device to another has been provided by the use of many different clock frequencies. However this solution also causes problems as it is difficult to optimise a delta modulator for a wide range of clock frequencies.